The present invention relates to drive tractors for web material. It finds particular application in the type of sheet-feed tractor used to feed paper in high-speed printers.
Paper is often fed to high-speed printers by tractors that have toothed belts that engage holes provided along either border of the paper sheet. The toothed belts are often trained around two wheels, a drive wheel and an idler wheel. It is important that the belts move synchronously and in tandem so that both edges of the paper are moved at the same rate and at the same time. For this reason, the drive wheels for both of the belts are typically driven by the same drive means so that equal simultaneous movement of the drive wheels is almost certainly assured. In many cases, this also assures equal simultaneous movement of the drive belts.
The advancement of technology and the explosion of the data processing field have required increasingly fast printing of data on the paper sheets. As the speed of printing increases, so does the speed of advancement of the paper sheet, to the extent that the inertia of the paper sheet and the drive belt becomes significant. As a result, it is possible for the sudden jerking movements of the drive wheel to stretch the drive belt, possibly allowing it to deflect outward from its normal path of travel by an amount sufficient to allow the belt to disengage from the drive wheel. It is apparent that disengagement of one belt can at the very least cause the paper sheet to be bent or torn, and it could possibly result in jamming of the feeder.
In order to avoid disengagement of the belt from the drive wheel, tension on the drive belt can of course be increased by various means. However, the amount of tension required to avoid most instances of disengagement increases as the required speed of motion increases, and this of course increases wear on the various moving parts. In addition, mere increase in tension does not provide a positive means for preventing the undesired disengagement.
Accordingly, what is required is a positive means for preventing disengagement of the drive belt from the drive wheel that can be inexpensively provided and that does not significantly increase wear on the moving parts.